Surface Mass Spectrometry
Emission of atoms and molecules from surfaces due to ion irradiation: Emission characteristics; surface ionization processes; compositional changes in multicomponent systems. The following graph depicts the dependence of the ionization probability on the surface work function. The exponential correlation agrees with the electron-tunneling model of ion formation.
Intensity as a function of work function change DF for C- and Si- secondary ions sputtered from graphite and silicon, respectively. The change of DF was effected by varying amounts of Cs in the near-surface region.
The detection of small doubly-charged molecular anions by means of highly sensitive mass spectrometry has been studied. The production of these gas-phase dianions is accomplished by sputtering the specimen with Cs+ ions with an energy of a few keV. It is demonstrated that dianions can be detected most easily when the molecular ion has an odd total mass; then, the dianions will show up at half-integral mass numbers in the mass spectrum. The flight time through the mass spectrometer of several µs establishes a lower limit with respect to the intrinsic lifetimes of these dianions.
Mass spectrum in the range of the doubly charged C102- cluster ion obtained by 14.5-keV Cs+ ion bombardment of a graphite specimen. A mass resolution of M/DM0.1 ~ 1400 was used to record this spectrum.
The existence of long-lived (metastable) molecular anions such as N2-, CO-, H2-, D2-, H3-, and D3- was demonstrated. These anion species were produced by sputtering of appropriate targets with Cs+ ions and were identified by high-sensitivity secondary-ion mass spectrometry. The figures below show exemplary mass spectra from these analyses.
High-resolution mass spectrum around 28 amu obtained by 14.5-keV Cs+ sputtering of a TiAlN(C,O) specimen. The arrows at the 28Si-peak mark the width of the 10%-intensity level; the resulting value DM0.1 = 0.00217 amu yields a mass resolution M/DM0.1 = 13000.
Mass spectra in the mass region m/q ~ 2 amu obtained by 14.5-keV Cs+ sputtering of HfH2. The H2- peak is separated from that of D- using high mass resolution (M/DM ~ 4000).
From the SIMS intensities, the following relative abundances were derived for the molecular hydrogen anions: H2-/H- = 3.5´10-6, H3-/H- = 1.7´10-8, D2-/D- = 3.1´10-5, D3-/D- = 7´10-9.
· Structural and electronic modifications of single crystal surfaces of semiconductors (e.g., Si, Ge, GaAs, InP) by low-energy ion bombardment. The diagrams show the ion-bombardment induced amorphization fluences for two semiconductor surfaces.
Amorphization fluence of Ge(100) as a function of Ar+ ion energy.
Amorphization fluence of GaAs(110) as a function of Ar+ ion energy. The parameter is the ion flux density.
· Micro-area ion implantation in various materials with focused ion beams.